In dustless abrasive blasting, an abrasive is entrained in a pressurized fluid flow or gaseous-entrained liquid flow and is directed against the surface to be treated by a controllable nozzle. It is the intention of such apparatus to coat each particle of the abrasive with the liquid so that the abrasive is weighted by the liquid and falls safely to the ground after striking the surface to be blasted, generally obviating the requirement for the operator of the apparatus to wear breathing apparatus. The weighted abrasive also increases the efficiency of the blasting operation. Typically, the liquid used in abrasive blasting apparatus is water or a water based blasting solution, such as a rust inhibiting solution. Similarly, the pressurized gaseous streams used in blasting operations are typically pressurized air.
Conventionally, a nozzle used in such apparatus comprises an inlet for attachment to the outlet hose of the blasting apparatus, an accelerating portion typically in the form of a venturi, and an outlet portion defining an outlet orifice through which the accelerated abrasive-laden gaseous jet is ejected from the nozzle. The outlet portion can take various forms and typically is conical in shape so that it flares uniformly towards the outlet orifice. This produces a circular blast pattern wherein the abrasive content is concentrated at the center of the jet. In effect, the abrasive distribution transversely across the cross-sectional axes of the outlet portion of the nozzle is in the form of a standard bell-shaped distribution curve. This means that in use, the operator of the apparatus must sweep the nozzle such that each sweep always overlaps at least a part of the previous sweep to produce an even blasting pattern on the surface to be cleaned. The effect of this is that much abrasive tends to be wasted by being blasted onto surfaces that have already been blasted previously and that do not need additional blasting.
In order to overcome this problem, different shaped nozzles are sometimes used. For example, the outlet portion of the nozzle can be made in the shape of a fan so that the outlet orifice is in the shape of a narrow rectangle. However, even in this case, the abrasive distribution transversely along the longitudinal axis of the outlet orifice is still a standard bell-shaped distribution curve. Likewise, nozzles with square-shaped outlet portions and outlet orifices have been proposed but without significantly altering the abrasive distribution within the final jet, wherein the abrasive content is always at its highest at the center of a jet.
The object of the present invention is to provide a blasting nozzle for use in an abrasive blasting apparatus that overcomes or substantially mitigates the aforementioned problem by altering the abrasive distribution pattern of the resulting jet when the nozzle is in use.